I think initially Orlandeau is slighltly stronger than Balthier.

But my hypothesis is that if you max them both out, Balthier will be stronger. Barrage does around 300 damage. Orlandeau does around 400 for me atm. But Orlandeau is slightly slower, even with haste, and he hasnt got as much range. But he can heal himself with shadowblade. The again, he doesnt have abilities such as arm shot and leg shot. And Balthier's plunder abilities make him the best thief in the game.

Once Balthier starts learning his best abilities, he's a more useful character IMO.